Method for the preparation of halide glass articles

ABSTRACT

Halide glass articles, e.g. rods, tubes and preforms for making fluoride glass fibres, are prepared by melting and/or casting the articles under a low pressure, e.g. 0.01 to 500 mbars and, during the low pressure regime, a gas flow rate of between 0.01 to 100 liters/min (measured at NTP) is maintained. It has been found that subjecting the melts to a low pressure reduces the attenuation of the fibre which eventually results from the melts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for the preparation of halide,preferably fluoride, glass articles, e.g., preforms suitable for drawinginto fibre.

2. Related Art

Halide, and especially fluoride, glass fibre is used where transmissionin the wavelength band 2000 nm to 4500 nm is required. Halide fibresalso display good transmission outside this band, e.g., over the range500 nm to 2000 nm but competitors, e.g., SiO₂ based fibres, have goodtransmission in this region. However, the competitors have such highattenuations in the band 2000 nm to 4500 nm that they are excluded forconsideration when it is required to use the fibre at this wavelength.

In most cases, the preparation of halide fibres involves first thepreparation of the core and clad glasses, the casting of these twoglasses to make a preform and the drawing of the preform into fibre. Itis important to avoid contamination during the preparation of theglasses and their casting. For this reason it is usual to carry outthese stages in isolation chambers which are provided with a dry inertatmosphere at slightly above the pressure outside the isolation chamber.The inert atmosphere is usually nitrogen for reason of cheapness butother inert gases, e.g., argon or helium, could also be used. It is alsoadvantageous to submit the melt to an oxidation process and mixtures ofinert gas and oxygen are used for this purpose. The transmissionproperties of a halide fibre are determined to a large extent bychemical considerations, e.g, the chemical composition of the core glassand the clad glass. It is also important that the two glass compositionscooperate to provide guidance and are compatible with one another duringthe preparative stages, especially the drawing.

The selection of the chemical compositions of the core and clad glassestogether with the careful preparative techniques indicated above areimportant to achieve low attenuation but it appears that mechanicalimperfections, e.g., crystals and bubbles, in the fibre can also causeattenuation, probably because mechanical imperfections can scatter thelight.

SUMMARY OF THE INVENTION

This invention is based upon the discovery that mechanical imperfectionscan be caused during the preparation of the glasses and in the castingof the preform and it has most surprisingly been discovered thatsubjecting the melts to treatment under the atmospheric conditionsspecified below, substantially reduces the incidence of mechanicalfaults whereby fibre with lower attenuation is achieved. It will beappreciated that subjecting solid glass to the specified treatment willhave little or no effect upon its properties and it is the moltenglasses which benefit from said treatment applied during the latterstages of their preparation and/or during casting. The melting of thehalide glasses is carried out in contact with atmospheres, especiallycontrolled atmospheres, which are conveniently provided in a chamberattached to an apparatus such as a glove box. The term "controlledatmosphere" includes inert atmosphere consisting of inert gases such asnitrogen, helium and argon. At certain stages of the process thecontrolled atmosphere may be pure oxygen or oxygen mixed with an inertgas. The atmospheric conditions mentioned above comprise a flow of gasat low pressure through the atmosphere which is in contact with themelt. Said flow of gas is preferably at a flow rate of 0.1 to 100liters/minute, e.g., 2 liters/minute as measured at NTP (normaltemperature and pressure). These correspond to ranges of 7×10⁻⁶ to7×10⁻³ moles/second and 1.5×10⁻⁴ moles/second respectively. Said lowpressure is preferably below 500 mbars (millibars), especially below 150mbar, e.g., within the range 5 to 150 mbars.

The invention is further defined in the claims.

During the casting of preforms, it is desirable that the pouring of thecore is carried out at a lower temperature and pressure than the pouringof the cladding, e.g., 20°-200° C. lower than the pouring of thecladding. The cladding is preferably poured at a pressure of below 500mbars, e.g., at a temperature at which is viscosity is 0.01 to 1000poise and under a pressure of 2-100 mbars. The preferred pressure forthe core is 0.01-2 mbars. It has been observed that these conditionsalso give good results even without the flow rates mentioned above.

The halide glasses mentioned above comprise (and preferably consist of)mixtures of metal halides wherein at least 90 mole %, and preferably 100mole % of the halide is fluoride. In the case where the percentage offluoride is less than 100%, it is preferred that the balance of thehalide is entirely chloride. Of the metals which constitute the halidespreferably at least 45 mole % is Zr and at least 10 mole % is Ba. It ispreferred that metals in addition to Zr and Ba are also present andthese are conveniently selected from Al, La, Na, Hf and Pb. The glasscomposition may also include dopants, e.g., rare earth metals such as Ndand/or Er to confer lasting properties on the glass. These dopants areconveniently present in the form of halides, especially fluorides.

The invention relates particularly to the preparation of halide (asdefined above) fibres, and especially fibres which are produced bydrawing preforms with a core/cladding structure. The preforms may bemade by casting a tube of a first halide glass, and, before the tubecools, casting a second halide glass into its bore. Alternatively, apreform may be assembled by casting tubes and rods, and shrinking thetubes onto the rods.

In addition, tubes may be shrunk onto preforms as described above. Thisis convenient for making preforms with more than 2 regions, e.g., usingmore than 2 different glass compositions and for making preforms withlarge cross sectional areas. It is also convenient to shrink tubes ontopreforms when it is desired to make fibre with small cores. This usuallyimplies a fibre in which the cross-sectional area of the cladding islarge in relation to the cross-sectional area of the core. Theconventional method, in which the core is cast into a tube of cladding,is mechanically difficult because of the small diameter of the tube.This difficulty can be avoided by casting a preform in which the size ofthe core precursor is convenient for casting. Stretching the preform sothat its diameter is reduced about 2-20 times, reduces the size of thecore but the preform no longer has an adequate diameter. Thereforeshrinking a tube of cladding glass onto the reduced preform restores theexternal dimension.

The glasses which are used for casting the articles previouslyidentified, i.e., tubes, rods and preforms, may be prepared by meltingtogether the appropriate fluorides or by fluorination of the appropriateoxides. These preparative methods are described in greater detail below.In addition the melts needed to cast the articles may be prepared bymelting previously formed glass compositions. In any case, however, theglass melt is prepared, the low pressure treatment specified above isapplied either to the melt immediately before casting, or to the meltduring casting, or during both stages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagram illustrating isolation chambers suitable for thepreparation of fibre preforms;

FIG. 2 is a diagrammatic illustration of a furnace suitable for meltinghalide glasses in accordance with the invention;

FIG. 3 is an illustration of a moulding box suitable for making preformsin accordance with the invention; and

FIG. 4 is attenuation curves comparing fibres prepared by differenttechniques.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The isolation chambers shown in FIG. 1 comprise a gas lock 10 forintroducing chemicals, crucibles and other equipment into the inertatmosphere. The gas lock 10 provides easy access to a storage chamber 11in which materials and equipment can be retained until required. Thestorage chamber 11 gives access to a preparation chamber 12 via a hatch(not shown). Crucibles 18 can be placed in the preparation chamber andcharges to make desired glass compositions are weighed into a crucible.(Where it is appropriate to distinguish, 18B will be used to denote acrucible used for cladding glass and 18A to denote a crucible used forcore glass).

The preparation chamber 12 gives direct access, via a hatch (not shown),to the casting chamber 13. A furnace 14 is located vertically above thecasting chamber 13. Said furnace is located in a furnace chamber 15 andits lower end extends into the casting chamber 13 for ease of access.The casting chamber 13 contains a moulding box 17 and it is providedwith a gas lock 16 which is used to take fibre preforms out of theisolation chambers. The furnace 14 is shown in greater detail in FIG. 2and the moulding box 17 is shown in greater detail in FIG. 3.

All the gas locks and chambers shown in FIG. 1 are provided with asupply of nitrogen (not shown) and with vents to allow used nitrogen togo to waste. The chambers are permanently flushed with dry nitrogen(less than 10 ppm of water) which is maintained at a pressure slightlyhigher than the external so as to reduce the risk of contaminationentering the isolation chambers.

The isolation chambers shown in FIG. 1 make it possible for chemicalsand other materials to be stored permanently in a dry atmosphere so thatthe risk of contamination is substantially reduced. In addition, allblending, melting and casting operations are carried out under a pure,dry atmosphere so that the risk of introducing impurities duringhandling is substantially reduced. Each chamber has its own separatesupply for the dry atmosphere and hatches are normally closed betweenthe chambers. Thus the risk of cross-contamination between the chambersis substantially reduced. It has been found that these precautions arenecessary because even slight contamination can substantially increasethe attenuation of fluoride glass fibres.

When crucibles 18 have been charged in the preparation chamber 12 (andlids applied to reduce the risk of contamination), they are transferredto the casting chamber 13 and introduced into the furnace 14. Theingredients are melted in the furnace 14 and the crucibles 18, completewith hot, molten charges are returned to the casting chamber 13. Whenthe preforms have solidified and cooled, they are removed via the gaslock 16 for drawing to fibre.

It should be noted that crucibles are usually processed in pairs, i.e.,one crucible 18A for the core glass and the other crucible 18B for theclad glass. More details of processing will be given below.

There are two basic processes for the preparation of fluoride glassesand the two processes will be described briefly. The isolation chambersshown in FIG. 1 are suitable for both processes.

According to the first process oxides of the selected metals are weighedin the preparation chamber 12 and mixed thoroughly in a crucible. Inaddition to the oxide powders ammonium bifluoride, NH₄ F₂ HF₂ isintroduced into the crucible. In the furnace 14 the ammonium bifluoridedecomposes and converts the oxide to fluorides.

In the alternative process the selected fluorides are weighed and mixedin the crucible. With this process it is not always necessary to use theammonium bifluoride because no chemical reaction is intended. However,as a precautionary measure, it is common to introduce a small amount ofammonium bifluoride into the crucible in case the fluoride powders arecontaminated by oxides.

It should be realized that hydroxide and oxide are the two mostobnoxious contaminants in fluoride fibres and, therefore, it isimportant to reduce the level of these contaminants to the minimum.

The furnace, shown as 14 in FIG. 1, comprises a body 20 which includesinsulation and electric heating elements. The body has an inlet port 23situated near the bottom and a vent 24 at the top.

The inlet port 23 is connected to a nitrogen mass flow valve 25 and anoxygen mass flow valve 26. These valves are adjustable during meltingoperation to provide a controlled and variable atmosphere duringprocessing. The vent 24 is connected to a controllable exhaust pump 27so that the pressure in the furnace can be varied. In addition, thefurnace is provided with a thermometer 28 for measuring the temperatureof operation.

For the convenience of the operatives the system includes amicroprocessor 29 which is operatively connected to:

the nitrogen mass flow valve 25,

the oxygen mass flow valve 26,

the exhaust pump 27,

the thermometer 28, and

the electric supply to the furnace.

The microprocessor 29 includes a timing means as well as a storagefacility for storing programs to operate the furnace for production runswhich programs include data defining an operational sequence. Thus themicroprocessor 29 provides automatic means for performing complicatedproduction schedules without the detailed attention of the operators.The microprocessor 29 can be programmed to emit a signal to attract theattention of operators when a production schedule has been completed.

During a melting schedule, the base of the furnace is closed by means ofa closure plate 19 which seals the furnace to prevent the contaminationof the furnace chamber 15 by gases evolved during heating. It isconvenient to support the crucibles 18A and 18B by means of the closureplate 19.

Before a melting operation crucibles 18A and 18B, at this stagecontaining the precursors of the glasses in the form of mixed powders,are introduced into the furnace and the closure plate 19 is applied toseal the furnace. (If it is intended to apply identical melt schedulesto both crucibles then it is convenient for both crucibles to go intothe same furnace. If different melt schedules are intended it isnecessary to use two furnaces.)

At this stage a supply of nitrogen, at ambient pressure, is passedthrough the furnace or furnaces. A typical melt schedule is as follows:

(1) In a controlled atmosphere, the crucibles 18A and 18B are raised tothe temperature at which any NH₄ F₂ HF₂ reacts. This temperature isusually in the range 200° to 500° C.

(2) The crucibles are maintained at this temperature for a period of 30to 90 mins to allow completion of any reactions which occur. A stream ofgas removes any vapours which are evolved during this stage.

(3) The temperature in the furnace is raised, e.g., to 700° to 900° C.,to ensure that all the components have passed into solution. This stageis conveniently carried out under the ambient pressure in the isolationchambers.

(4) The melts are oxidised for 10 to 150 minutes. Oxygen, preferably inthe form of blends containing O₂ and inert gas in the mole ratios 1:0 to1:100, was used as the oxidising agent.

(5) When the temperature of the glass has reduced by at least 50° C. thesetting on the nitrogen flow valve 25 is reduced to zero whilstcontinuing with the flow of oxygen and the exhaust pump 27 started. Thisreduces the pressure in the furnace to about 50 mbar. This has theeffect that the last stage of the melting is carried out under reducedpressure in accordance with the invention. The atmosphere may be pure O₂at this stage. The temperature should be at least 600° C.

(6) The supply of nitrogen is re-started and the supply of oxygen isterminated. When the oxygen is clear of the furnace the closure plate 19is removed and the crucibles are transferred into the casting chamber13.

The structure and use of the casting box 17 will now be described.

The oxygen treatment, i.e., step (4), is described in our patentportfolio consisting of EP 170380, U.S. Pat. No. 4,741,752, U.S. Pat.No. 4,848,997 and CD 1267537.

After removal from the furnace 14 the hot crucibles 18A and 18B,containing molten glass, are placed in the casting box 17 for theformation of the preform.

The casting box 17 has a lid 30 which opens to allow the entry of thecrucibles 18A and 18B. When the lid 30 is closed the pressure in the box17 can be reduced. The box 17 contains a conventional mould for thecentrifugal casting of tubes. This comprises a tubular mould 31 whichcan be rotated about its longitudinal axes to provide the centrifugalforce for casting and which can be tilted between the horizontal and thevertical. Since this is a conventional arrangement for the centrifugalcasting of tubes, it will not be described in detail. The box 17 alsocontains manipulators 32 and 33 and it also has a vent 34 for connectionto suction to reduce the pressure.

After melting in the furnace, the hot crucibles 18A and 18B aretransferred into the holders 32 and 33 and the lid 30 is closed. At thisstage suction can be applied to the vent 34 so that the pressure in thebox 17 is reduced. Preferably the box 17 is not completely sealed sinceit is desired to cause a constant flow of nitrogen through the boxduring the casting process. Suitable flow rates are 0.01 to 100liters/min, preferably 0.1 to 10 liters/min, (as measured at NTP). Thepressure in the box and the temperature of the glass vary during thecasting process. These variations will be briefly described.

Initially the pressure is reduced below 500 mbars, preferably below 150mbars, preferably to a pressure of 2-100 mbars. Under a pressure withinthis range the manipulator 33 is used to pour the cladding glass intothe mould 31 and, when it is poured, the viscosity of the cladding glassis preferably 0.1 to 1000 poise. (This often requires a temperature inthe range of 450°-600° for lasers with more than 50 mole % ZrF₄). Afterpouring, the mould is rotated about its longitudinal axis and thiscauses the molten glass to be distributed evenly around the mould 31 sothat a tube is formed. At this stage the temperature must be high enoughso that the glass is sufficiently mobile to form a good tube but thehigh temperature makes it desirable to maintain the pressure above aminimum determined by the temperature of the glass. This avoidsvapourisation of volatile components which could re-condense andcontaminate the preform. Even trivial amounts of re-condensation cancause substantial defects by nucleating crystal growth. Thus attemperatures close to 500° C. it would be undesirable to use pressuressubstantially below 100 mbars. Because of the chilling caused by themould 31 the cladding glass is cooled and it solidifies. The core glassin the crucible 18A cools, but not as quickly as the cladding glass inthe mould 31, so that the core glass remains mobile. When the claddingglass is sufficiently solidified the rotation is terminated and thelongitudinal axis of the mould is tilted back to the verticleorientation. At this point the manipulator 32 is used to pour the coreglass from the crucible 18A into the bore of the tube which has justbeen formed. This operation is preformed at a temperature which is lowerthan the temperature at which the cladding glass was poured, e.g., at atemperature which is 20°-200° C. lower. It will be appreciated that thelower temperature means that the core glass will have a substantiallyhigher viscosity than the cladding glass during pouring but it has beenfound that this still allows good casting of the core to be achieved. Atthese lower temperatures, the pressure in the system can be furtherreduced so that the core glass is cast at a pressure below that at whichthe cladding glass was cast. Conveniently the core glass is cast underpressure of 0.01 to 2 mbars. The casting of the core completes thecasting operation and the preform is allowed to cool under a reducedpressure.

When the preform has cooled enough to be handled, the suction at vent 34is terminated, the pressure in the box 17 allowed to return to theambient pressure in the casting chamber 13, and the preform is annealedin the mould. When it has cooled the mould 31, containing the solidpreform, is now removed from the isolation chambers via the gas lock 16.

The preform, which has been prepared in accordance with the inventionbecause the last stages of the melting and the casting were carried outunder reduced pressure, is converted into fibre using conventionaltechniques. The preform may be drawn in the form in which it was castbut improved fibre performance may be obtained by the use of one or moreof the following features.

Polishing

Poor surface quality can sometimes impair the strength of fibre andincrease the loss. Therefore, it may be desired to polish the surface ofthe preform preliminary to drawing. The polishing may be carried outmechanically using abrasives.

Etching

Gentle chemical etching, e.g., using a solution of ZrOCl₂ to removesurface layers which may be contaminated. Etching is often appropriateto remove abrasives which have been used in a previous polishing stage.

Ion Bombardment

It has been found that placing the preform in a vacuum chamber andbombarding it with suitable ions can remove a very thin surface layer.This is valuable where contamination is limited to very thin surfacelayers and the use of a vacuum reduces the risk of recontamination.

Protective Coating

As a final treatment before drawing, it is often convenient to apply aprotective coating to the surface of the preform. Chalcogenide glasseshave been recognised as a particularly suitable form of coating becausethey form barrier layers to protect the fluoride fibre from a hostileenvironment. Very thin layers of chalcogenide glass can be applied byion beam sputtering in the same vacuum chamber which is used forbombardment. This allows at least a preliminary coating to be applied tothe preform while it is still under vacuum and before exposure to theair allows recontamination. (One form of ion bombardment and coating aredescribed in our patents EP 266889 and U.S. Pat. No. 4,863,237).

Finally, using conventional techniques, the preform is drawn so that itsdiameter is reduced in the ratio (20-220):1, e.g., 80:1, so as toproduce the fibre which is the ultimate product of the invention. It hasbeen demonstrated that the use of low pressures as described above canreduce the attenuation of the fibre by a factor of about 10. It can alsoimprove the mechanical strength of the resulting fibres.

Three fluoride glass fibres were prepared by three different methods.Each of the fibres had the composition specified in the following table.

    ______________________________________                                        Component    Core Glass                                                                              Cladding Glass                                         ______________________________________                                        ZrF.sub.4    58.6      62.1                                                   BaF.sub.2    23.2      24.6                                                   LaF.sub.3    7.1       5.6                                                    AlF.sub.3    1.9       1.8                                                    NaF          5.1       5.9                                                    PbF.sub.2    4.1       0                                                      ______________________________________                                    

wherein the numbers represent percentage by weight.

A more detailed description of these fibres is given in our patents EP170380 and U.S. Pat. No. 4,836,643.

Fibre A was prepared by the preferred embodiment of the invention usinga low pressure treatment during both melting and casting.

Fibre B was also prepared according to the invention but using the lowpressure treatment during melting only.

Fibre X was prepared according to the prior art without any low pressuretreatment.

FIG. 4 shows the attenuation for all three fibres and it is easy to seethat the low pressure treatment according to the invention substantiallyreduces the attenuation.

The minimum attenuation is at about 2700 nm in each case and theattenuations are:

    ______________________________________                                        Fibre A      1.5 dB/km                                                        Fibre B      6.5 dB/km                                                        Fibre X      20.5 dB/km.                                                      ______________________________________                                    

As should be apparent from the use of absolute SI units throughout thespecification and claims, all pressures are stated in absolute unitsrather than gauge units.

We claim:
 1. Method of preparation of a halide glass article whichmethod comprises:preparing a halide glass melt, casting the glass melt,and treating the melt of halide glass with a flow of gas at a pressureless than atmospheric during the preparation of the melt or during thecasting of said melt or during both said preparation and said castingwherein at least 90 mole % of the halide of the said halide glass isfluoride.
 2. A method according to claim 1, wherein the pressure iswithin the range 0.01 to 500 millibars.
 3. A method according to claim2, wherein the pressure is within the range 0.01 to 100 millibars.
 4. Amethod according to claim 1, wherein the flow of gas is maintained at agas flow rate of between 0.01 to 100 liters/min as measured at normaltemperature and pressure.
 5. A method according to claim 4, wherein theflow rate is between 0.1 and 10 liters/min.
 6. A method according toclaim 1, wherein the gas comprises oxygen.
 7. A method according toclaim 1, wherein the treating step includes casting of said melt carriedout with a dry inert atmosphere.
 8. A method according to claim 1,further comprising:(a) preliminarily melting halide glass in thepresence of a dry atmosphere at ambient pressure; and (b) contacting themelted glass at ambient pressure with a dry atmosphere being eitheroxygen or a mixture of oxygen and inert gas, said atmosphere containingoxygen and inert gas in the mole ratio O₂ : inert gas=1:0 to 1:100.
 9. Amethod according to any one of the preceding claims, wherein all of thehalide of said halide glass is fluoride.
 10. A method according to claim9, wherein the fluoride glass consists essentially of(a) at least onefluoride selected from the group consisting of the fluorides of Zr andBa and (b) at least one fluoride selected from the group consisting ofthe fluorides of Na, Al, La, Pb, and Hf.
 11. A method according to claim1, wherein the article is a preform for drawing into fibre, said preformcomprising a core of one halide glass and a cladding of a differenthalide glass and wherein each of the two glasses is prepared and castwith a gas flow at a pressure less than atmospheric pressure.
 12. Amethod of preparing a halide glass fibre, which method comprisespreparing a preform by a method according to claim 11 and thereafterdrawing said preform into fibre.
 13. A method according to claim 1wherein all of the halide of said halide glass melt is fluoride.
 14. Amethod of preparing a fluoride glass fibre which fibre comprises a coreof a second fluoride glass surrounded by and in contact with a claddingof a first fluoride glass wherein the refractive index of the secondfluoride glass is higher than the refractive index of the first fluorideglass, which method comprises:(a) providing the precursors of said firstfluoride glass into a first crucible, (b) providing the precursors ofsaid second fluoride glass into a second crucible, (c) transferring eachof said crucibles into a furnace and subjecting both cruciblessimultaneously to a melting regime comprising, in the order specified,(c1) pre-heating said crucibles in the presence of a dry gas at ambientpressure until any evolution of gas has ceased, (c2) heating saidcrucibles to a temperature above the melting point of the glasses in thepresence of a mixture of oxygen and inert gas in a mole ratio 1:0 to1:100, (c3) reducing the temperature to below 500 millibars andmaintaining a gas flow of 0.01 to 100 liters/min (as measured at normaltemperature and pressure), (d) transferring the crucibles to a castingzone and casting said first glass composition as a tube and casting saidsecond glass composition into the bore of the said tube whereby a fibrepreform is produced, both of said castings being conducted under apressure of less than 150 millibars and a flow rate of 0.01 to 100liters/min (as measured at normal temperature and pressure), and (e)drawing the fibre preform into a fibre.
 15. A method of preparing ahalide glass fibre which method comprises:casting a cladding precursormelt as a tube and thereafter casting a core precursor melt into thebore of said tube, the cladding precursor melt being cast into a mouldunder a flow of gas at a pressure less than atmospheric pressure, thecore precursor melt being cast into said bore under a pressure which islower than the pressure at which the cladding precursor melt was cast tothereby form a preform, the core precursor melt being cast while at asecond temperature which is lower than a first temperature at which thecladding precursor melt was cast and, drawing said preform into a fibre.16. A method according to claim 15, in which the cladding precursor meltis cast at a pressure below 500 millibars.
 17. A method according toclaim 15 in which the core precursor melt is cast at said secondtemperature which is 20°-200° C. lower than the first temperature atwhich the cladding precursor melt was cast.
 18. A method according toclaim 17, wherein the cladding precursor melt is cast at said firsttemperature at which its viscosity is 0.01 to 1000 poise and under apressure of 2-100 millibars.
 19. A method according to claim 17 whereinthe core precursor melt is cast under a pressure of 0.01 to 2 millibars.20. A method of preparing a fluoride glass fibre which fibre comprises acore of a second fluoride glass surrounded by and in contact with acladding of a first fluoride glass wherein the refractive index of thesecond fluoride glass is higher than the refractive index of the firstfluoride glass, which method comprises:(a) providing precursors of saidfirst fluoride glass into a first crucible, (b) providing precursors ofsaid second fluoride glass into a second crucible, (c) transferring eachof said crucibles into a furnace and subjecting both cruciblessimultaneously to a melting regime comprising, in the order specified,(c1) pre-heating said crucibles in the presence of a dry gas at ambientpressure until any evolution of gas substantially ceases, (c2) heatingsaid crucibles to a temperature above the melting point of the glassesin the presence of a mixture of oxygen and inert gas in a mole ratio offrom 1:0 to 1:100, (c3) reducing the temperature by at least 50° C. butnot to a temperature below 600° C. and causing a flow of gas throughsaid furnace at a pressure in the range 5-150 millibars, (d)transferring the crucibles to a casting zone and, (d1) casting saidfirst fluoride glass as a tube wherein said casting includes pouring themelted precursors of said first fluoride glass into a mould at atemperature at which its viscosity is 0.01 to 1000 poise under apressure of 2-100 millibars, (d2) casting said second fluoride glassinto the bore of said tube whereby a fibre preform is produced, whereinthe pouring of the melted precursors of said first fluoride glass intobore is carried out at a temperature which is at least 20° C. lower thanthe temperature used in step (d1) and under a pressure in the range of0.1-2 millibars, and (e) drawing the fibre preform into a fibre.
 21. Amethod of preparing halide glass articles which methodcomprises:preparing a melt of a halide glass and thereafter shaping saidmelt into a halide glass article, at least 90% mole of the halide ofsaid halide glass being fluoride, and after said melt has been preparedand before it is shaped into said article, treating said melt with aflow of gas at less than atmospheric pressure.
 22. A method for thepreparation of a halide glass article which methodcomprises:simultaneously preparing plural melts of halide glass andthereafter casting said melts to thereby form an article, at least 90%mole of the halide of said halide glass being fluoride, and performingsaid casting in the presence of a flow of gas at less than atmosphericpressure.